1. Field of the Invention:
The present invention relates generally to adhesives and/or sealants which are used in bonding or sealing metal structures. More particularly, the present invention relates to providing an adhesive and/or sealant which can be inspected for voids after being applied between metal structures.
2. Description of Related Art:
The bond between metal structures achieved with adhesives such as epoxies is, in many cases, better than the bonds achieved using conventional fastening methods such as rivets or screws. Although metal bonding was initially pioneered in aerospace applications, this technology is now being used in a wide range of applications where bonding of metal structures and/or sealing is required. Exemplary applications in addition to aerospace uses include marine applications, military applications, automotive applications and rail applications.
The use of polymeric adhesives to bond metal structures together provides a number of advantages. These advantages include more uniform distribution of stress loads, lightweight bonding, bond strengths equivalent to conventional fastening procedures, and the simultaneous formation of airtight seals while bonding is achieved. A wide variety of polymeric adhesives/sealants are presently available for use in such applications. The majority of the presently available adhesives are epoxy or urethane based materials. These adhesives usually contain a resin, which is a mixture of monomers and polymers, a curing agent, various modifiers and solid filler materials. The amount and kind of solid filler materials present in the adhesive generally determines the bond strength of the material. The filler decreases the brittleness of the adhesive via a crack-stopping mechanism. The adhesives or sealants are supplied as kits which may have some of the ingredients premixed. Immediately prior to use, the curing agent is added to the mixture and then the adhesive is applied to the work surfaces. The surfaces are positioned for bonding and the adhesive is allowed to cure. Alternatively, the mixture can be frozen and its application delayed, i.e., a frozen premix.
In order to ensure optimum metal bonding and/or sealing, it is essential that there be no voids in the adhesive layer. Voids or air channels can occur during the bonding/assembly process. The effectiveness of sealing is reduced by defects such as bubbles, incomplete knit lines or cavities in the adhesive. Injection bonding involves introducing the liquid adhesive mixture into the adhesive joint cavity using external pressure. The flow path of the adhesive may cause entrapped air or result in incomplete knit-lines. It is therefore essential that a method of inspection be devised which can detect such voids in the adhesive layer after assembly of the metal structures.
Exotic methods of non-destructive inspection, such as neutron radiography, are available. However, such exotic inspection methods are extremely expensive and not of practical use. X-ray radiography inspection is a readily available and well-known non-destructive inspection technique. However, the conventional adhesives utilized in metal bonding are not visible by x-ray radiography when surrounded by metal. Accordingly, there is presently a need to provide a simple and inexpensive means for allowing non-destructive testing of metal bonded and/or sealed structures to detect the presence of voids or incomplete knit-lines in the adhesive/sealants.